1. Field of the Invention
This invention generally relates to an image sensing device, and more particularly to an image sensing device with microlenses.
2. Description of Related Art
Recently, because data processing speed and data storage capacity are greatly improved in an image capturing chip, the performance in graphic processing has been enhanced dramatically. Hence, image processing devices such as scanners, digital cameras, and digital camcorders become indispensable in our daily life. Further, those devices become smaller and smaller for our convenience.
Generally, an optical scanner includes a light source on the chassis of the scanning module. The scanning module includes a set of reflectors, a lens and an image sensing device inside the chassis. The scanning module can scan documents one-on-one by linear transmission mechanism. Further, when a light source emits a light to the document, the image of the document can be formed on the image sensing device by focusing via the lens and reflecting via the reflectors. The image sensing device can be a charge-coupled device (CCD) or a CMOS device.
FIG. 1 is the side view of the scanning module of a conventional optical scanner. FIG. 2 shows the light path in the conventional scanning module. Referring to FIG. 1, the scanning module 100 includes a chassis 110, a light source 112, a plurality of reflectors 114, 116, and 118, a lens 120, and an image sensing device 122. The light source 112 can be a fluorescent lamp or an LED array on the top surface of the chassis 110. The light source 112 emitsa light to a scan-pending document. The reflectors 114, 116, and 118, the lens 120, and the image sensing device 122 are inside the chassis 110. When the light source 112 emits the light to the document, the image of the document is formed on the image sensing device 122 by focusing via the lens 120 and reflecting via the reflectors 114, 116, and 118. Referring to FIG. 2, the distance S1 between the document 10 and the lens 120 is the so-called object distance, while the distance S2 between the lens 120 and the image sensing device 122 is the so-called image distance. The amount of S1 and S2 is the so-called total trace TT. The image of the document 10 is reflected by the reflectors 114, 116, and 118 to project a light cone on the lens 120. Then the lens 120 focuses the image to form another light cone on the image sensing device 122.
It should be noted that the image sensing device includes a plurality of linear sensing cells (or sensing chips) 124, 126. Each sensing cell represents a pixel. Hence, the more number of the sensing cells, the higher the resolution of the image sensing device per unit area. However, when the resolution is higher, the required luminous flux has to be increased in order to obtain a sharper image. The conventional way is to apply a plurality of microlenses 134 and 136 on each sensing cell 124 and 126 to further focus the image in order to increase the luminous flux.
FIG. 2A is an enlarged view illustrating how the incident lights enter into the microlenses. Because the sensing cells 124 and 126 are linearly arranged and the outer area of the sensing cell 126 has a longer light path than the central area of the sensing cell 124, the incident light cannot enter into the microlens 136 at the outer area perpendicularly. Instead, the incident light enters into the microlens 136 at an incident angle. When the incident angle is larger than or equal to the total reflection angle, a portion of lights is totally reflected at the surface of the microlens 136 or be deflected, and thus cannot enter into the sensing cell 126. Therefore, there is an insufficient luminous flux at the edge of the image. Further, after the incident light is further focused by the microlens 136, the luminous flux projected into the sensing cell increases. However, the light may shift slightly to affect the quality of the image. When the image distance S2 between the lens 120 and the image sensing device 122 is getting smaller, the possibility to total reflection at the two sides of the image sensing device 122 would become higher, which causes a serious image distortion to the image signal of the document 10.